1. Field of the Invention
The present invention relates to a photo film cassette of a leader-advancing type. More particularly, the present invention relates to a leader-advancing photo film cassette of which an undeveloped or developed status of photo film can be discerned.
2. Description Related to the Prior Art
A widely used 135-type of photo film cassette contains photo film of which a leader is protruded from a cassette shell in an unexposed status. During insertion of the cassette into a camera, the leader of the photo film as well as the cassette shell require attention for handling. It is difficult remarkably for an unskilled amateur to handle the photo film cassette in loading of the camera. There is a newer type of photo film cassette, of which a leader of unexposed photo film is pre-contained in the cassette shell, and advanced to an outside of the cassette shell when a spool is rotated in an unwinding direction. Such a type of cassette is suggested in U.S. Pat. Nos. 4,832,275 and 4,834,306, and a commonly assigned U.S. Pat. No. 5,213,277 (corresponding to JP-A 3-267932).
Also, there is a cartridge for containing a developed photo film having the same structure as the above type, as disclosed in U.S. Pat. No. 5,083,155 (corresponding to JP-A 3-179341). All the developed photo film can be wound into the cassette shell, and kept without being cut. When required, the spool is rotated to advance a leader of the developed photo film, before a desired frame is pulled out by adjusting a pulled amount of the photo film. The handling and the preservation of the developed photo film is facilitated. It is also easy to automate optical instruments including a photographic printer, a video printer and the like, because operability of those can be simplified.
It is conceivable to use a cassette shell for unexposed photo film also as a cartridge for a developed photo film. However it is impossible visually to discern whether the photo film is developed or undeveloped while the photo film is wound in the cassette shell, because the leader is wholly contained in the cassette shell. Various errors are likely to occur: an unexposed photo film, or an exposed and undeveloped photo film is likely to be unwound out of a cassette; and the photo film after the exposure and the development is likely to be loaded in a camera.
To solve this problem, it is known to form a tongue with a certain position outside a cassette shell. If the tongue is folded and eliminated, this indicates that the development has been effected.
If the user erroneously grasps or pushes the tongue in holding the cassette, it is likely that the tongue is folded and separated easily. It is conceivable to raise the thickness of the tongue to strengthen it. In turn such a thick tongue has a drawback of impossibility in folding. Otherwise the thick tongue may be folded in an unwanted shape, to have a rough shape at its fold.
Recently public concern has been shown on environmental problems. When the tongue is folded and eliminated from the cassette shell, a problem of waste of the tongue piece arises. More seriously, the tongue as separated is likely to drop in a camera, and cause breakdown of the camera.